Effects of agricultural management on Vertosols in Tasmania

Attributes of 21 Vertosols in 2 different regions of Tasmania were assessed using field and laboratory techniques to determine differences associated with 3 local forms of agricultural management ( long-term pasture, rain-fed cropping, and irrigated cropping). Vertosols in the northern Midlands had better physical properties ( lesser bulk density and penetration resistance, and greater porosities and water holding capacities), poorer nutrient status ( lower pH, exchangeable bases, and extractable P), and better biological properties ( greater organic carbon (OC), carbon fractions F1 and F3, and more worms) than south-eastern Vertosols. When adjusted for clay content, cropped sites had less soil OC than pasture sites at 0-75 mm depth. Readily oxidisable ( fraction F1) carbon in the surface 75 mm was 3.6 mg/g and 6.9 mg/g under long-term pasture compared with 2.5 mg/g and 3.9 mg/g in irrigated cropped paddocks on south-eastern and Midlands sites, respectively. Soil organic carbon values were positively correlated with physical and chemical soil properties. Long-term pasture paddocks showed stronger structural development and had smaller aggregates than cropped paddocks, which had more larger clods. Vane shear strength and penetration resistance were less in rainfed cropped paddocks compared with long-term pasture but this effect was not apparent on irrigated cropped paddocks. Farmers considered that a majority of their soil attributes were healthy under all management histories but strategies for maintaining organic matter levels and minimising clod formation by tillage are essential for long-term sustainable use of these Vertosols.